Oscillator, and receiving device and electronic device using the oscillator

ABSTRACT

An oscillator unit is configured such that a frequency adjustment unit of a synthesizer used by a controller is smaller than a frequency variation tracking capability of a demodulator connected to an output side of a frequency converter. This structure successfully combines the temperature compensation control of an oscillator unit and the receiving process of a high-frequency receiving device. Accordingly, an oscillator unit with large temperature coefficient is applicable to high-frequency receiving devices.

This Application is a divisional of U.S. patent application Ser. No.12/671,340, filed on Jan. 29, 2010, which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to oscillator units that produceoscillation signals, and receiving devices and electronic devices usingthe oscillator unit.

BACKGROUND ART

FIG. 7 is a circuit diagram of a conventional reference oscillator. Inreference oscillator 100 in FIG. 7, oscillator 101 is, for example, anAT-cut quartz crystal. Driver circuit 102 connected in parallel tooscillator 101 is configured, for example, with a CMOS inverter. Loadcapacitances 103 and 104 are connected to oscillator 101 and ground.

In general, the reference oscillator used for a reference frequency ofcommunications devices, such as mobile phones, or high-frequencyreceiving devices, such as TV sets, require frequency stability againstambient conditions. In particular, frequency stability againsttemperature changes is one important performance. For example, TV setsrequire stability of at least ±60 ppm or less is required in a usetemperature range. A structure of reference oscillator 100 is effectivefor achieving this performance, and crystal oscillator 101 is anessential device for equipment requiring highly accurate frequencystability.

However, crystal oscillator 101 has a structure of suspending anoscillating portion in midair while holding a part of crystal piece cutto a predetermined shape. Accordingly, downsizing is difficult. Inaddition, a device having the above structure needs to be manufacturedone by one. This makes cost reduction difficult.

To redeem the disadvantage of reference oscillator 100 made of crystal,an oscillator using a silicon oscillator utilizing a semiconductormanufacturing process has been disclosed. A reference oscillatoremploying a silicon oscillator is configured in the same way as that inFIG. 7. However, since a temperature coefficient of silicon material islarge, an oscillation frequency varies in line with a temperaturechange. Therefore, a temperature sensor is used for detecting a changein ambient temperature so as to apply temperature compensation controlfor retaining a constant frequency.

FIG. 8 is a block diagram of a conventional oscillator unit. In FIG. 8,conventional oscillator unit 201 includes reference oscillator 202 forgenerating a reference oscillation signal, synthesizer 204 foroutputting a local oscillation signal based on the reference oscillationsignal output from this reference oscillator 202, temperature sensor 205for detecting temperature, and controller 206. Controller 206 adjusts afrequency of the local oscillation signal output from synthesizer 204based on a detection result of temperature sensor 205. This controller206 applies temperature compensation control for adjusting outputfrequency of synthesizer 204 based on a temperature detection result ofreference oscillator 202 detected by temperature sensor 205. This priorart is disclosed, for example, in Patent Literature 1.

A temperature coefficient of silicon oscillator (not illustrated) inreference oscillator 202 is 30 ppm/° C., which is large. Therefore, afrequency adjustment level output from controller 206, corresponding tothe detection result of temperature sensor 205, becomes large. As aresult, a frequency variation in the local oscillation signal outputfrom synthesizer 204 becomes large.

On the other hand, in a high-frequency receiving device, a frequency ofhigh-frequency signal received is converted to an intermediate frequencysignal, using the local oscillation signal obtained by converting asignal output from the oscillator. This is demodulated in a laterprocess. Stable frequency without variation is thus demanded for thisintermediate frequency signal. Accordingly, in case of using theoscillator in the high-frequency receiving device, a demodulator may notbe able to demodulate if frequency greatly varies in the intermediatefrequency signal as a result of temperature compensation control.Therefore, the oscillator unit using an oscillator with largetemperature coefficient cannot be used in a field of high-frequencyreceiving devices, such as mobile phones and broadcast receiving tuners,even if a temperature compensation control circuit is added.

Patent Literature 1: U.S. Pat. No. 7,145,402

SUMMARY OF THE INVENTION

The present invention offers an oscillator unit that is applicable tohigh-frequency receiving devices even if a temperature coefficient islarge.

In the oscillator unit of the present invention, frequency adjustmentunit fstep of a synthesizer used by a controller is smaller thanfrequency variation tracking capability fv of a demodulator connected toan output side of a frequency converter.

This structure successfully combines the temperature compensationcontrol of the oscillator unit and the receiving process of ahigh-frequency receiving device. Accordingly, an oscillator unit withlarge temperature coefficient is applicable to high-frequency receivingdevices.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a block diagram of a receiving device equipped with anoscillator unit in accordance with a first exemplary embodiment of thepresent invention.

FIG. 1B is a block diagram of an electronic device equipped with thereceiving device in accordance with the first exemplary embodiment ofthe present invention.

FIG. 2 is a block diagram of a receiving device equipped with anoscillator unit in accordance with a second exemplary embodiment of thepresent invention.

FIG. 3 is a block diagram of a receiving device equipped with anoscillator in accordance with a third exemplary embodiment of thepresent invention.

FIG. 4 is a block diagram of a receiving device equipped with anoscillator unit in accordance with a fourth exemplary embodiment of thepresent invention.

FIG. 5 illustrates changes by time of an output of a temperature sensorand an integrator in the oscillator unit in accordance with the fourthexemplary embodiment of the present invention.

FIG. 6 is a block diagram of a receiving device equipped with anoscillator unit in accordance with a fifth exemplary embodiment of thepresent invention.

FIG. 7 is a circuit diagram of a conventional reference oscillator.

FIG. 8 is a block diagram of a conventional oscillator unit.

REFERENCE MARKS IN THE DRAWINGS

1 Oscillator unit

2 Reference oscillator

3 Frequency converter

4 Synthesizer

5 Temperature sensor

6 Controller

7 Receiver

8 Demodulator

9 Receiving device

10 Difference detector

11 M-frequency divider

12 Phase detector

13 N-frequency divider

14 VCO

15 Filter

16 Integrator

17 Computing unit

18 AD converter

19 Second frequency converter

20 Processor

21 NCO

36 First frequency converter

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS First Exemplary Embodiment

The first exemplary embodiment of the present invention is describedbelow.

FIG. 1A is a block diagram of a receiving device equipped with anoscillator unit in the first exemplary embodiment of the presentinvention. In FIG. 1A, oscillator unit 1 includes reference oscillator 2for producing a reference oscillation signal, synthesizer 4, temperaturesensor 5 for detecting temperature, and controller 6. Synthesizer 4produces a local oscillation signal based on the reference oscillationsignal output from reference oscillator 2, and outputs this localoscillation signal to frequency converter 3. Controller 6 adjusts afrequency of the local oscillation signal output from synthesizer 4based on a detection result of temperature sensor 5.

Other than oscillator unit 1, receiving device 9 equipped with thisoscillator unit 1 includes receiver 7 for receiving a high-frequencysignal; frequency converter 3 connected to oscillator unit 1 andreceiver 7, and producing an intermediate frequency signal; anddemodulator 8 for demodulating an intermediate frequency signal.

In this structure, controller 6 calculates a frequency adjustment level,using a temperature detection result of temperature sensor 5, andcontrols synthesizer 4. As a result, frequency variations in synthesizer4 can be suppressed within a predetermined range in a predeterminedtemperature range even if a temperature coefficient of an oscillatorincluded in reference oscillator 2 is large. For example, when atemperature coefficient of the oscillator is 30 ppm/° C., a frequencyvariation will be 3600 ppm in a temperature range from −40° C. to 80° C.if there is no control using the above temperature sensor. This meansthat a frequency variation is as large as 3600 kHz, if the output ofsynthesizer 4 is, for example, 1 GHz (hereafter, the output ofsynthesizer 4 is “1 GHz” in the description). On the other hand, if anappropriate control is applied using temperature sensor 5, the variationcan be suppressed, for example, to 0.6 ppm/° C. or below (60 kHzconverted to the output of synthesizer 4). This enables suppression offrequency variation to 72 ppm (±36 ppm, 0.6 ppm/° C.×120° C.) in thetemperature range from −40° C. to 80° C. However, if oscillator unit 1is used for configuring a high-frequency receiving device, receptionbecomes not feasible (reception error) for a period of a fewmilliseconds or a few seconds in a reception period.

This happens as described below. In oscillator unit 1, controller 6outputs a frequency adjustment level based on a detection result oftemperature sensor 5. Accordingly, if a rate of resonance frequencyvariation is large with respect to the temperature of oscillator (notillustrated) included in reference oscillator 2 (hereafter referred toas a “frequency temperature coefficient”), the frequency adjustmentlevel output from controller 6 dependently becomes large.

For example, let's say a resonator is a silicon oscillator. A frequencytemperature coefficient of the silicone oscillator is 30 ppm/° C., andthus a resonance frequency varies for 3 ppm when the temperature changesfor 0.1° C. at a certain moment. Temperature sensor 5 detects thistemperature, and transmits this information to controller 6. Then, basedon the information from controller 6, synthesizer 4 shifts the frequencyfor 3 ppm.

In the first exemplary embodiment, an output signal of synthesizer 4 isinput to frequency converter 3. However, the output signal ofsynthesizer 4 may be sent to a second synthesizer (not illustrated), andan output signal of the second synthesizer may be input to frequencyconverter 3. In both cases, a frequency of local oscillation signalinput to frequency converter 3 will suddenly change for 3 ppm at acertain moment.

If a frequency of receiving signal output from receiver 7 is 500 MHz, afrequency variation of 3 ppm is equivalent to 1.5 kHz. This immediatelybecomes a change in intermediate frequency signal output from frequencyconverter 3. In other words, the intermediate frequency signal input todemodulator 8 changes for 1.5 kHz due to frequency adjustment bycontroller 6. Demodulator 8 cannot track this change instantaneously,and results in “reception error.” Then, after a few moments, AFC (AutoFrequency Control) inside demodulator 8 activates, and reception isrecovered. The time until this recovery differs by the structure ofdemodulator 8.

However, the reception error does not always occur by the frequencyvariation of intermediate frequency signal. If this frequency variationis below a predetermined value, the reception error does not occur. Inother words, demodulator 8 has a tolerance against instantaneousfrequency variation (this tolerance is referred to as “frequencyvariation tracking capability,” and expressed as “fv”).

The first factor that determines frequency variation tracking capabilityfv of demodulator 8 is a Doppler tolerance of demodulator 8 (hereafterthe Doppler tolerance is expressed as “fd1.”). Doppler tolerance fd1 isalso called Rayleigh fading tolerance, and is mostly dependent on systemmodulation and demodulation principles or waveform equalizationprinciple in internal processing of demodulator 8.

In general, system modulation and demodulation principles with largeDoppler tolerance sacrifice a data transmission speed. In order toimprove the Doppler tolerance also in demodulator 8, a circuit size isincreased or a trade-off with degradation in white noise is oftenneeded. For example, if Doppler tolerance fd1 of receiving device 9 usedin the first exemplary embodiment is about 100 Hz, demodulator 8 candemodulate an input signal even if receiving device 9 moves at a speedof about 100 km per hour on receiving a frequency of about 1 GHz.

A frequency variation in the receiving signal caused by the Dopplereffect becomes a frequency variation in the intermediate frequencysignal output from frequency converter 3. Accordingly, if this amount issmaller than fd1, demodulator 8 can demodulate the input signal. Inprevious steps before demodulator 8, a frequency variation in localoscillation signal input to frequency converter 3 is equivalent to avariation in the intermediate frequency signal output from frequencyconverter 3. Accordingly, even if a frequency variation momentarilyoccurs in the local oscillation signal, demodulator 8 can demodulate theinput signal if this variation is smaller than fd1.

The second factor that determines frequency variation trackingcapability fv of demodulator 8 is a tracking capability ofaforementioned AFC. AFC detects and corrects frequency deviation ofintermediate frequency signal input to demodulator 8, based on a periodof receiving signal output from receiver 7. For example, in digitalbroadcast in Japan, a broadcast signal is configured with a symbol unitof about 1-milisecond period. This period is called a “symbol period.”Demodulator 8 converts the intermediate frequency signal to a basebandsignal using internal NCO (Numerical Controlled Oscillator), which isnot illustrated. Then, aforementioned symbol period is extracted byusing a guard interval signal specified by digital broadcast in Japan.An initial value of NCO is set to a predetermined frequency ofintermediate frequency signal. However, if frequency deviation exists inthe intermediate frequency signal, a frequency deviation occurs in abaseband signal output from NCO, and thus aforementioned symbolfrequency deviates. Therefore, a value set to NCO is corrected so thatan extracted symbol period becomes a predetermined value. This correctsdeviation in intermediate frequency signal.

A detectable and correctable frequency range (pull-in range) offrequency deviation in the intermediate frequency signal can be madebroad (e.g. several tens of kHz). However, to stabilize frequencycorrection by AFC, integral treatment needs to be applied to correctionfor each receiving signal period. This makes a tracking time long withrespect to the receiving signal period. Accordingly, reception erroroccurs until AFC completes tracking. The reception error occurs iffrequency variation in local oscillation signal caused by temperaturecompensation control in oscillator unit 1 exceeds fd1, and receptionbecomes feasible again after a tracking time, which is determined byweight parameters of integral treatment in AFC.

Frequency variation tracking capability fv of demodulator 8 isdetermined mainly by the aforementioned factors, and depends on systemmodulation and demodulation principles and a structure of high-frequencyreceiving device including demodulator 8.

Therefore, the oscillator unit of the present invention focusesattention on this frequency variation tracking capability fv. Controller6 controls synthesizer 4 using a frequency adjustment unit (hereafterreferred to as “fstep”) smaller than fv. The fstep refers to a minimumadjustable level for digitally adjusting oscillation frequency. Thismakes the frequency variation level of intermediate frequency input todemodulator 8 smaller than frequency variation tracking capability fv.Demodulator 8 has tolerance to this frequency variation, and thus makesit feasible to eliminate reception error caused by the temperaturecompensation control. In addition, since frequency variation trackingcapability fv≦Doppler tolerance fd1 is at least established, occurrenceof reception error can be suppressed by setting at least frequencyadjustment unit fstep smaller than Doppler tolerance fd1, even iffrequency variation tracking capability fv cannot be determined at theinitial design stage.

Furthermore, frequency adjustment without delay against a temperaturechange in reference oscillator 2 can be achieved by satisfying arelational expression of Formula 1, whereas N is the number of frequencyadjustment per unit time in synthesizer 4, ΔF is a frequency variationlevel per unit temperature in local oscillation signal, and ΔT is atemperature change per unit time in reference oscillator.

N×fstep>ΔF×ΔT  (Formula 1)

A case of using the above example is detailed next. Since a frequencyvariation level of reference oscillator 2 per 1° C. is 30 ppm, outputfrom synthesizer 4 becomes 30 kHz per 1° C. (ΔF=30 kHz). If atemperature changes for 0.1° C. per second (ΔT=0.1° C.), a frequencyvariation of 3 kHz occurs. Here, if frequency adjustment unit fstep is100 Hz, Formula 1 is satisfied by setting N>30. More specifically, byconfiguring controller 6 to adjust frequency in a period of 30 times ormore per second, the frequency can be adjusted without any demodulationerror in demodulator 8 and also without any delay against thetemperature change. This structure thus simplifies a device, inparticular, a controller, without any complicated control.

FIG. 1B is a block diagram of an electronic device equipped with thereceiving device in the first exemplary embodiment of the presentinvention. In FIG. 1B, electronic device 50 includes receiving device 9,display 51 connected to an output of receiving device 9, transmittingdevice 52 disposed near receiving device 9, and battery 53 for supplyingpower to the receiving device. For example, if this receiving device 9is applied to a mobile phone, one of electronic devices, oscillator unit1 of the present invention increasingly demonstrates the above effect.Display 51, transmitting device 52, and battery 53 have large influenceon the ambient temperature change by their operation and non-operation.For example, if the receiving device in the first exemplary embodimentis fully used independently, a temperature change during its use is 0.05to 0.1° C./second. However, when the receiving device is installed in acommunications device, a temperature changes from 0.1 to 0.3° C./second.Its influence is more than doubled. Accordingly, frequency variation perunit time in the output of synthesizer 4 becomes greater when oscillatorunit 1 is installed in an electronic device with display 51. If thefrequency variation in the output of synthesizer 4 becomes large,greater frequency adjustment level is consequently needed every time,increasing the probability of causing reception error. Duration ofreception error also becomes longer. In the first exemplary embodiment,however, adjustment is applied while retaining the relation offstep≦fd1. Therefore, the receiving process that does not depend on thetemperature change per unit time, and without any reception errorbecomes feasible. At the same time, the use of Formula 1 makes itfeasible to adjust frequency against the temperature change withoutdelay, further increasing the effect of the first exemplary embodiment.Furthermore, the electronic device can be downsized and cost reductionis achievable by providing an input terminal of reference oscillationsignal to receiving device 9 and using a reference oscillator outsidethe receiving device.

As described above, receiving device 9 of the present inventionincreases its effect when receiving device 9 is installed in electronicdevice 50. Still more, the oscillator unit of the present invention iseffective as an oscillator unit for a transmitting device, in additionto the receiving device. If a frequency variation in a transmittingdevice also occurs due to temperature compensation control in theoscillator unit, the aforementioned problem also occurs in the receivingdevice receiving a transmission signal from this transmitting device.Accordingly, communications without transmission and reception errorsbecome feasible by applying frequency adjustment, which is needed inline with a temperature change in the oscillator unit in thetransmitting unit, in the unit smaller than Doppler tolerance of thereceiving device.

The first exemplary embodiment uses an oscillator unit with a frequencytemperature coefficient of 0.6 ppm/° C. or below. However, a requiredfrequency temperature coefficient changes in line with a requiredfrequency variation level in a high-frequency receiving device. In thefirst exemplary embodiment, the oscillator unit with 0.6 ppm/° C. isadopted because a target frequency stability of the local oscillator inthe high-frequency receiving device is set to ±40 ppm, considering amargin against desired stability of ±60 ppm. An oscillator unit withfurther better frequency stability is applicable. As another example, ifthe target frequency stability is set to ±6 ppm, the oscillator unitneeds to be controlled to achieve at least 0.1 ppm/° C.

Still more, in the first exemplary embodiment, an operating temperaturechange is 0.05 to 0.1° C./second. However, this change depends on theenvironment where the device is used and a structure of the deviceitself. Accordingly, each device is designed appropriate for each case.Controller 6 is not necessarily included in the oscillator unit. It maybe included in demodulator 8 in line with a specific structure ofreceiving device 9. Alternatively, a microcomputer connected to asubsequent step may be used for control.

Furthermore, the first exemplary embodiment refers to a structure ofdirectly connecting synthesizer 4 producing a signal input to thefrequency converter and reference oscillator 2. However, if variation inoscillation frequency due to the temperature of reference oscillator 2is large against system requirements, a first synthesizer is providedafter the reference oscillator, a second synthesizer using this outputas a reference signal is provided, and an output from the secondsynthesizer may be input to frequency converter 3. If a filter isprovided to the output signal of frequency converter 3, a cutofffrequency of filter may be adjusted when controller 6 adjusts thefrequency. This prevents the filter from attenuating all or part of areceiving signal band even if the frequency of output signal fromfrequency converter 3 varies.

Second Exemplary Embodiment

The second exemplary embodiment of the present invention is describedbelow.

FIG. 2 is a block diagram of a receiving device equipped with anoscillator unit in the second exemplary embodiment of the presentinvention. In FIG. 2, oscillator unit 1 includes reference oscillator 2for producing a reference oscillation signal, synthesizer 4, andcontroller 6 for adjusting an output frequency of synthesizer 4.Synthesizer 4 produces a local oscillation signal, based on thereference oscillation signal output from reference oscillator 2, andoutputs this local oscillation signal to frequency converter 3.Receiving device 9 equipped with this oscillator unit 1 includesoscillator unit 1, receiver 7 for receiving a high-frequency signal,frequency converter 3 connected to oscillator unit 1 and receiver 7 forgenerating an intermediate frequency signal, demodulator 8 fordemodulating the intermediate frequency signal, and difference detector10 for detecting a difference between an output of frequency converter 3and a predetermined intermediate frequency. An output of this differencedetector 10 is connected to controller 6. Alternatively, differencedetector 10 may detect a difference between an output of referenceoscillator 2 or an output of synthesizer 4 and a predetermined frequencycorresponding to each of them.

In this structure, controller 6 adjusts an output frequency ofsynthesizer 4 such that an output of difference detector 10 becomes 0.As described in the first exemplary embodiment, a demodulation erroroccurs if an amount exceeding frequency variation tracking capability fvof demodulator 8 is adjusted instantaneously. Therefore, frequencyadjustment unit fstep of synthesizer 4 is set to a value smaller than fvso that demodulation error can be prevented. The frequency ofsynthesizer 4 can be adjusted, following output frequency variationagainst a predetermined intermediate frequency, by satisfying arelational expression of Formula 2, whereas N is the number of frequencyadjustment per unit time in synthesizer 4, ΔF is a frequency variationper unit temperature in the local oscillation signal, and d is adifference per unit time output from difference detector 10.

N×fstep>d  Formula 2

Receiving device 9 in the second exemplary embodiment eliminates theneed of temperature sensor 5, and thus oscillator unit 1 can be furtherdownsized.

Third Exemplary Embodiment

The third exemplary embodiment is described below.

FIG. 3 is a block diagram of a receiving device equipped with anoscillator unit in the third exemplary embodiment of the presentinvention. In FIG. 3, oscillator unit 1 has the same structure as thatof the first exemplary embodiment. Synthesizer 4 includes M-frequencydivider 11 connected to an output of reference oscillator 2; phasedetector 12 connected to this M-frequency divider 11 and an output ofN-frequency divider, which is described later; filter 15 connected to anoutput of phase detector 12; VCO 14 connected to an output of thisfilter 15; and N-frequency divider connected to an output of this VCO14. An output terminal of controller 6 is connected to N-frequencydivider. Temperature sensor 5 is connected to controller 6. As in thesecond exemplary embodiment, difference detector 10 may be connected tocontroller 6.

In this structure, M-frequency divider 11 divides the output ofreference oscillator 2 to 1/M frequency, and N-frequency divider 13divides the output of VCO 14 to 1/N frequency. Phase detector 12compares output phases of M-frequency divider 11 and N-frequency divider13, and controls frequency of VCO 14 to achieve 0-phase difference.Frequency fout of local oscillation signal output from synthesizer 4 isdetermined in accordance with Formula 3 by setting frequency-dividingratio M of M-frequency divider and frequency-dividing ratio N ofN-frequency divider.

Fout=(N/M)×fin  Formula 3

Here, fin is a frequency of output signal from reference oscillator 2.Fractional numbers may also be set to frequency-dividing ratio N.

For example, if fin is 20 MHz, frequency-dividing ratio M is 2, andpredetermined frequency adjustment unit fstep is 100 Hz; the minimumsetting unit of 10⁻⁵ (=100×2/(20×10⁶) can be set to frequency-dividingratio N.

As described above, the output frequency of synthesizer 4 can be changedin units smaller than frequency variation tracking capability fv ofdemodulator 8 by appropriately designing the setting unit forfrequency-dividing ratio N and/or frequency-dividing ratio M. Afrequency-divider may be disposed between synthesizer 4 and frequencyconverter 3, depending on a structure of receiving device 9. An objectof the present invention is also achieved by controlling thisfrequency-divider using the fstep unit.

Still more, a large time constant may be set to filter 15 on adjustingfrequency by controller 6. This enables suppression of variation in thevoltage input to VCO 14 within a predetermined range. As a result,variation in output frequency of synthesizer 4 can be suppressed withina predetermined range.

Furthermore, the output frequency of synthesizer 4 can also be changedby switching capacitance of VCO or controlling offset value of controlvoltage applied to VCO. In these cases, the output frequency ofsynthesizer 4 can be changed in the fstep unit by setting appropriateswitching unit for capacitance or appropriate resolution power foroffset value of control voltage.

Fourth Exemplary Embodiment

The fourth exemplary embodiment of the present invention is describedbelow.

FIG. 4 is a block diagram of a receiving device equipped with anoscillator unit in the fourth exemplary embodiment of the presentinvention. In FIG. 4, oscillator unit 1 has the same structure as thatin the first exemplary embodiment. Controller 6 includes integrator 16connected to an output of temperature sensor 5, and computing unit 17connected to an output of integrator 16. Next is described how frequencyis adjusted such that frequency adjustment unit fstep of synthesizer 4becomes smaller than frequency variation tracking capability fv ofdemodulator 8 in this structure.

FIG. 5 illustrates changes over time of output of the temperature sensorand the integrator of the oscillator unit in the fourth exemplaryembodiment of the present invention. In FIG. 5, temperature sensor 5outputs a predetermined voltage against a temperature detected by asensor (not illustrated). In general, the temperature sensor tracks atemperature change early enough, and thus the output voltage fromtemperature sensor 5 follows a dotted line in FIG. 5 when thetemperature suddenly changes. In other words, the output voltage(vertical axis) suddenly rises with respect to time (horizontal axis).In controller 6, computing unit 17 converts this output voltage to apredetermined frequency adjustment level, and controls synthesizer 4.However, if the temperature suddenly changes, the frequency is alsocontrolled to change suddenly, and demodulator 8 may not be feasible totrack this change. Accordingly, in the fourth exemplary embodiment, asuddenly rising output voltage is controlled to a moderate change sothat demodulator 8 can track, when a sudden temperature change occurs,by making this output voltage pass through integrator 16 with apredetermined time constant. In other words, frequency adjusted bysynthesizer 4 per unit time is kept below fstep. This adjustment makesfrequency adjustment unit fstep of synthesizer 4 smaller than frequencyvariation tracking capability fv of demodulator 8. A solid line in FIG.5 is a control signal waveform output from controller 6.

This integrator may be configured with an analog circuit including aresistor, capacitor, inductor, OP amplifier, and so on. Or, the outputvoltage of temperature sensor 5 may be converted to digital output by anA/D converter and then apply digital computing for integration. Stillmore, integrator 16 and computing unit 17 may be disposed in theopposite order so as to apply integration to frequency adjustment level,which is an output of computing unit 17. This also achieves the sameobject.

Furthermore, a memory (not illustrated) and comparator (not illustrated)may be provided between integrator 16 and computing unit 17 so as toupdate the output of computing unit 17 only when a difference between anoutput voltage of integrator 16 and a previous output voltage stored inthe memory exceeds a predetermined threshold. This eliminates thefrequency adjustment against a faint temperature change, stabilizing anoutput frequency of synthesizer 4.

In the structure described above, an appropriate time constant set to anintegration circuit enables stabilization of output frequency ofsynthesizer 4 with a simple circuit.

The above refers to the case of connecting temperature sensor 5.However, this concept is applicable to the structure of using differencedetector 10 in the second exemplary embodiment. Also in this case,frequency is adjusted only when a difference between the output ofintegrator connected to an output of difference detector 10 and aprevious output of integrator stored in the memory exceeds apredetermined threshold. This stabilizes the output frequency ofsynthesizer 4.

Fifth Exemplary Embodiment

The fifth exemplary embodiment of the present invention is describedbelow.

FIG. 6 is a block diagram of a receiving device equipped with anoscillator unit in the fifth exemplary embodiment of the presentinvention. In FIG. 6, oscillator unit 1 has the same structure as thatin the first exemplary embodiment. Demodulator 8 includes AD converter18 connected to an output of first frequency converter 36, NCO 21 foroutputting a frequency equivalent to an intermediate frequency, secondfrequency converter 19 connected to outputs of AD converter 18 and NCO21, and processor 20 connected to an output of second frequencyconverter 19. An output of controller 6 is connected to NCO 21 insteadof synthesizer 4.

With this structure, controller 6 can stabilize the output frequency ofsynthesizer 4 also by controlling NCO 21 in accordance with thefrequency adjustment condition described in the first exemplaryembodiment. Since frequency variation tracking capability fv ofdemodulator 8 depends on processor 20, an adjustment of output frequencyof synthesizer 4 in accordance with a predetermined condition isequivalent to an adjustment of output frequency of NCO 21 in accordancewith a predetermined condition for processor 20.

In the above first to third exemplary embodiments, specific values aregiven to facilitate description. However, a technical feature of thepresent invention against the prior art is that a frequency variationcaused by a temperature coefficient of oscillator in the referenceoscillator is adjusted according to a condition determined by thefrequency variation tracking capability of demodulator. Accordingly, theoscillator unit of the present invention is applicable to frequencybands used in a range of communications system and specific structuresinside receiving devices.

INDUSTRIAL APPLICABILITY

The oscillator unit, receiving device, and electronic device of thepresent invention enable temperature compensation control thatsuppresses reception errors by adjusting frequencies in the minimum unitset based on values within an allowable range of frequency variationtracking capability of the demodulator. This allows the use of MEMS(Micro Electro Mechanical System) oscillator made of silicon materialwith large temperature coefficient for high-frequency receiving deviceswith small frequency variation tracking capability. Accordingly, thepresent invention contributes to downsizing and cost reduction ofelectronic devices, such as mobile terminals and broadcast receivers.

1. A receiving device comprising: an input terminal of a referenceoscillation signal; a synthesizer for producing a local oscillationsignal based on the reference oscillation signal from the inputterminal; a receiver for receiving a signal; a frequency converter forconverting a frequency of an output signal from the receiver by usingthe local oscillation signal output from the synthesizer; a demodulatorfor demodulating a signal output from the frequency converter; adifference detector for detecting a difference between a frequency of asignal and a predetermined frequency, the signal being output from atleast one the input terminal, the synthesizer, and the frequencyconverter, and a controller for adjusting a frequency of the localoscillation signal output from the synthesizer based on a detectionresult of the difference detector; wherein a frequency adjustment unitof the synthesizer used by the controller is smaller than a frequencyvariation tracking capability of the demodulator.
 2. An electronicdevice comprising: the receiving device of claim 1; a decoder connectedto an output side of the demodulator; and a display connected to anoutput side of the decoder.
 3. An electronic device comprising: thereceiving device of claim 1; a decoder connected to an output side ofthe demodulator; and a display connected to an output side of thedecoder.